System for measuring temperature of a cryogenic environment

ABSTRACT

A system for measuring the temperature of a cryogenic environment including an RC circuit provided with a source of reference voltage with a resistance sensor disposed within the environment and connected to a trigger. A pulser is connected to supply the RC circuit and a temperature indicator is connected to a pulser and trigger to count the RC interval for determination of the temperature of the environment.

United States Patent Hartman [451 June 27, 1972 [54] SYSTEM FORMEASURING TEMPERATURE OF A CRYOGENIC ENVIRONMENT [72] Inventor: RichardL. Hartman, Huntsville, Ala.

[73] Assignee: The United States of America as represented by theSecretary of the Army 22 Filed: Jan. 26, 1971 21 Appl.No.: 109,876

[52] U.S. Cl. ..73/362 AR, 73/362 R 51 Int. Cl ..G0lk 7/24, GOlk 7/34[58] Field oi Search ..73/362 R, 362 CP, 362 AR [56] References CitedUNITED STATES PATENTS 3,477,292 11/1969 Thornton ..73/362 AR 3,620,082 11/1971 Peters ..73/362 AR Primary Examiner-Louis R. Prince AssistantExaminer-Frederick Shoon Auorney-Harry M. Saragovitz, Edward J. Kelly,Herbert Berl and Charles R. Carter ABSTRACT 2 Claims, 2 Drawing Figures3| PULSE GENERATOR TIME INTERVAL COUNTER PATENTfoJunzl m2 3.672218 3|PULSE T 33a TIME GENERATOR TRIGGER INTERVAL COUNTER l4 FIG. I

OR 330 v TRIGGER REFERENCE I /voLTAcE 33b I I I I 34 I I ON OFF FIG. 2

Richard L. Hartman,

INVENTOR.

SYSTEM FOR MEASURING TEMPERATURE OF A CRYOGENIC ENVIRONMENT BACKGROUNDOF THE INVENTION This invention relates to the field of measurement ofcryogenic temperatures in small volume environments.

Temperature readings of contemporary direct current (DC) devices are inerror due to thermal EMF's or generation of heat in junctions betweendissimilar metals. The heat generated in the time required to balancethe measuring devices of the current systems is sufficient to cause asignificant increase in the temperature and consequent error in thetemperature indication. Alternating current (AC) measuring devicesgenerate errors due to continuous induction between members of thecircuit.

SUMMARY OF THE INVENTION The claimed system eliminates the errors ofboth current systems to measure cryogenic temperatures with power lessthan 1 micro watt. A pulser is connected to an RC circuit to operate atrigger having a reference voltage. An intervalometer measures the timefor the pulse to operate the trigger. If the reference voltage is set ata particular fraction of full value the counter reads the time intervalof one time constant (RC). Thus if C is known, R can be determined. If Chas a decimal value the counter reads directly a decimal multiple of R.The induction error and the energy build up due to the time required tobalance the DC system are thus avoided. The thermal EMFs are not onlyreduced by elimination of the balancing time but the percent thermal EMFerror is fractionated due to the high level of the pulse voltage.

This invention may be better understood from the following detaileddescription taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a schematic of the measuringcircuit. FIG. 2 is a chart of the timing sequence.

DESCRIPTION OF THE PREFERRED EMBODIMENT Reference numeral indicates agenerator for applying high level voltage pulses to the measuringsystem. An RC circuit consists of a sensor 13 located in a cryogenicenvironment 14 and a capacitor I9. The resistance of sensor 13 decreaseswith temperature. The RC circuit is connected to the input of a trigger15 that in turn is connected to a variable potentiometer 21 forsupplying a reference voltage thereto. Thus it is seen that the pulsersupplies an indigenous reference to the trigger.

A time interval counter 17, which acts as a temperature indicator. isconnected by a lead from the pulse generator for receiving the initialturn on pulse from generator 10 and is also connected to the output oftrigger 15 for receiving a trigger output signal to turn off thecounter.

When the pulse generator is turned on, an initial pulse shown in FIG. 2as 31 will turn counter on as indicated in FIG. 2 by reference numeral34. Simultaneously the pulse will impress a value on the trigger input33b from the pulse generator through variable potentiometer 21. Also thepulse is applied across resistor 13 to charge capacitor 19, the voltagerises exponentially with the time constant of the RC circuit as shown by32 in FIG. 2. The trigger compares the voltage across the capacitor frominput 33a with the voltage set by potentiometer 21 and when they areequal, the trigger sends an output signal to turn off the counter asseen at 34b in FIG. 2.

With the reference voltage set at substantially 0.632 of full value, thecounter records the time interval of one pulse which is equal to thecircuit time constant RC (the time to charge capacitor 19). If C isknown, R can be determined and if C has a decimal value, e.g. l.0 ufd,the scale of the time interval counter 17 can be calibrated to readtemperature by determination of the readings for a particular resistor13 at 4.2"

Kelvin the temperature of liquid helium, at 2 1 Kelvin the temperatureof liquid hydrogen and at 77 Kelvin the temperature of liquid nitrogenand can be interpolated therebetween. The current through the resistordrops to zero as the capacitor charges therefore only a given incrementof energy is dissipated in the resistor for each pulse, regardless ofhow long the pulse is. If the pulse interval is made much longer thanthe time constant, the power in the system is reduced by this dutyfactor. A 10 Mhz (megahertz) clock in the counter affords a four digitmeasurement and may be regulated to sample the temperature once persecond. Each reading takes less than I millisecond giving a duty factorof one one-thousandth. The duty factor can be selected to limit thepower dissipated in the resistor to a value that will not materiallyeffect the temperature of the environment.

I claim:

1. A system for measuring the temperature of a cryogenic environmentcomprising: a resistor-capacitor (RC) circuit having atemperature-responsive resistor disposed within the environment andconnected between a trigger and a pulser which supplies power to the RCcircuit; an indigenous source of reference voltage, said triggerresponding to a voltage across the RC circuit equal to said referencevoltage, and a temperature indicator connected to the pulser and triggerto count the RC interval for determination of the temperature of theenvironment.

2. A system as defined in claim 1 with said source of reference voltagecomprising a grounded variable potentiometer connected between saidpulser and said trigger.

1. A system for measuring the temperature of a cryogenic environmentcomprising: a resistor-capacitor (RC) circuit having atemperature-responsive resistor disposed within the environment andconnected between a trigger and a pulser which supplies power to the RCcircuit; an indigenous source of reference voltage, said triggerresponding to a voltage across the RC circuit equal to said referencevoltage, and a temperature indicator connected to the pulser and triggerto count the RC interval for determination of the temperature of theenvironment.
 2. A system as defined in claim 1 with said source ofreference voltage comprising a grounded variable potentiometer connectedbetween said pulser and said trigger.